Computer Networks

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What is a computer network

● A network consists of two or more computers that are linked in order to share resources , exchange files, or allow electronic communications.

● The computers on a network may be linked through cables, telephone lines, radio waves, satellites, or infrared light beams.

Need of computer networks

Resource sharing➢ Peripheral Sharing➢ Data sharing➢ Co-operative Computation➢ Sharing of Services ➢ Communication

Examples of networks

➢ Internet➢ College network➢ Americal on line

Network topology➢ A network may be represented as a collection of

nodes, some of which are connected by links.➢ A given node may have links to many others. ➢ Network topology is determined only by the

configuration of connections between nodes; it is therefore a part of Graph theory.

➢ Distances between nodes, physical interconnections, (Communication by means of transmitted signals) transmission rates, and/or (Any communication that encodes a message) signal types are not a matter of network topology, although they may be effecton actual physical network.

Network topology (cont.)Evaluation Criteria➢ - Network performance and speed➢ - Network reliability and redundancy➢ - Security of the network➢ - Acquisition and support costs➢ - Criteria of the network bandwidth

being met➢ - Flexibility and enhanced future

bandwidth capabilities

Network Topology (cont.)● Complete or fully connected ● Star● Ring● Bus● Tree● Line

Complete Topology● A fully connected or complete topology is a

network topology in which there is a direct link between all pairs of nodes. In a fully connected network with n nodes, there are n(n-1)/2 direct links.

● Synonym fully connected mesh network.

Complete topology

Star topology

A network topology in which peripheral nodes are connected to a central node, which rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, including the originating node. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only.

Star topology (cont.)

Ring Topology

A network topology in which every node has exactly two branches connected to it. These nodes and branches form a ring. If one of the nodes on the ring fails then the ring is broken and cannot work. A dual ring topology has four branches connected to it, and is more resistant to failures.

Ring topology (cont.)

Linear Bus topologyBus topology is such that there is a

single line to which all nodes are connected, and the nodes connect only to this bus.

Bus topology (cont.)

Layered Approach➢ The OSI reference model is a hierarchical structure

of seven layers that defines the requirements for communications between two computers.

➢ The model was defined by the International Standards Organization.

➢ It was conceived to allow interoperability across the various platforms offered by vendors. The model allows all network elements to operate together, regardless of who built them. By the late 1970's, ISO was recommending the implementation of the OSI model as a networking standard; unfortunately, TCP/IP had been in use for years.

➢ Only a subset of the whole OSI model is used today.➢ It is widely believed that much of the specification is

too complicated and its full functionality has taken too long to implement, although there are many people that strongly support the OSI model.

Purpose of OSI● The OSI model divides the functions of a protocol

into a series of layers.● Each layer has the property that it only uses the

functions of the layer below, and only exports functionality to the layer above.

● A system that implements protocol behavior consisting of a series of these layers is known as a 'protocol stack' or 'stack'.

● Protocol stacks can be implemented either in hardware or software, or a mixture of both.

● Typically, only the lower layers are implemented in hardware, with the higher layers being implemented in software.

Physical Layer➢ The physical layer defines all electrical and physical

specifications for devices. This includes the layout of pins, voltages, and cable specifications.

➢ Hubs and repeaters are physical-layer devices. The major functions and services performed by the physical layer are:

o Establishment and termination of a connection to a communications medium.

o Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.

o Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel.

Data link layerThe Data link layer provides the functional and

procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. The addressing scheme is physical which means that the addresses are hard-coded into the network cards at the time of manufacture. The addressing scheme is flat. Note: The best known example of this is Ethernet. Other examples of data link protocols are HDLC and ADCCP for point-to-point or packet-switched networks and LLC and Aloha for local area networks. This is the layer at which bridges and switches operate.

Network Layer➢ The Network layer provides the functional

and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the Transport layer.

➢ The Network layer performs network routing, flow control, segmentation/desegmentation, and error control functions.

➢ The router operates at this layer -- sending data throughout the extended network and making the Internet possible, although there are layer 3 (or IP) switches.

➢ This is a logical addressing scheme - values are chosen by the network engineer.

➢ The addressing scheme is hierarchical.

Transport Layer➢ The purpose of the Transport layer is to

provide transparent transfer of data between end users, thus relieving the upper layers from any concern with providing reliable and cost-effective data transfer.

➢ The transport layer controls the reliability of a given link. Some protocols are stateful and connection oriented. This means that the transport layer can keep track of the packets and retransmit those that fail. The best known example of a layer 4 protocol is TCP.

Session Layer➢ The Session layer provides the

mechanism for managing the dialogue between end-user application processes.

➢ It provides for either duplex or half-duplex operation and establishes check pointing, adjournment, termination, and restart procedures. This layer is responsible for setting up and tearing down TCP/IP sessions.

Presentation layer➢ The Presentation layer relieves the

Application layer of concern regarding syntactical differences in data representation within the end-user systems. MIME encoding, encryption and similar manipulation of the presentation of data is done at this layer. An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.

Application Layer➢ This layer interfaces directly to and performs

common application services for the application processes. The common application services provide semantic conversion between associated application processes. Examples of common application services include the virtual file, virtual terminal (for example, Telnet), and "Job transfer and Manipulation protocol" (JTM, standard ISO/IEC 8832).

The OSI model in the reality

➢ Real-world protocol suites often do not strictly match the seven-layer model. There can be some argument as to where the distinctions between layers are drawn; there is no one correct answer. However, most protocol suites share the concept of three general sections: media, covering layers 1 and 2; transport, covering layers 3 and 4, and application, covering layers 5 through 7.

➢ The DoD model, developed in the 1970s for DARPA, is a 4-layer model that maps closely to current common internet protocols. It is based on a more "pragmatic" approach to networking than OSI.

Interfaces➢ In addition to standards for individual

protocols in transmission, there are also interface standards for different layers to talk to the ones above or below (usually operating-system-specific).

➢ Example: Microsoft Windows' Winsock and Unix's Berkeley sockets and System V Streams are interfaces between applications (layers 5 and above) and the transport (layer 4). NDIS and ODI are interfaces between the media (layer 2) and the network protocol (layer 3).

Examples of LayersLayer 7 (Application) : HTTP , FTP , NFS , NTPLayer 6 ( Presentation) : SSL ,TLSLayer 5 (Session) : NetBios, TCP session

establishment.Layer 4( Transport) : NetBEUI, TCP, UDPLayer 3( Network) : NetBEUI, IP, ICMP, IPsec,

ARP.Layer 2(Data Link): Ethernet, Token Ring,

PPP, HDLC, Frame Relay, ATM, Fibre Channel

Layer 1( Physical ) RS-232, V.35, V.34, T1, 10BASE-T, 100BASE-TX, ISDN, DSL

Communicaion Methods➢ A method of routing traffic between an

originator and a destination through switching centers, from local users or from other switching centers, whereby a continuous electrical circuit is established and maintained between the calling and called stations until it is released by one of those stations.

➢ The method of establishing the connection and monitoring its progress and availability may utilize a separate control channel as in the case of ISDN or not as in the case of the Public Switched Telephone Network.

Example – switching techniques

➢ Circuit Switching➢ Message Switching➢ Packet Switching

Circuit Switching➢ Circuit switching is the most familiar

technique used to build a communications network.

➢ It is used for usually for telephone networks.

➢ It allows communications equipment and circuits, to be shared among users.

➢ Each user has sole access to a circuit (functionally equivalent to a pair of copper wires) during network use.

Circuit switching (cont.)➢ Consider communication between two points

A and D in a network. The connection between A and D is provided using (shared) links between two other pieces of equipment, B and C.

Circuit switching (cont.)Network use is initiated by a connection phase,

during which a circuit is set up between source and destination, and terminated by a disconnect phase. These phases, with associated timings.

Message SwitchingSometimes there is no need for a circuit to be

established all the way from the source to the destination. Consider a connection between the users (A and D) in the figure below (i.e. A and D) is represented by a series of links (AB, BC, and CD).

Message switching (cont.)➢ For instance, when a telex (or email) message is

sent from A to D, it first passes over a local connection (AB). It is then passed at some later time to C (via link BC), and from there to the destination (via link CD).

➢ At each message switch, the received message is stored, and a connection is subsequently made to deliver the message to the neighboring message switch.

➢ Message switching is also known as store-and-forward switching since the messages are stored at intermediate nodes en route to their destinations.

Message Switching (cont.)Most message switched networks do not use

dedicated point-to-point links and therefore a call must be set-up using a circuit switched network.

Packet Switching➢ Packet switching is similar to message

switching using short messages. ➢ Any message exceeding a network-defined

maximum length is broken up into shorter units, known as packets.

➢ For transmission; the packets, each with an associated header, are then transmitted individually through the network.

➢ The fundamental difference in packet communication is that the data is formed into packets with a pre-defined header format (i.e. PCI), and well-known "idle" patterns which are used to occupy the link when there is no data to be communicated.

Packet Switching (cont.)

Types of networks LANs - Local Area Network, a computer network that

spans a distance of tens of metres at the most. LANs are very common in offices but can also connect several offices together.

WANs - Wide Area Network, a computer network that spans hundreds of metres to a few kilometres. Typically the campus spanning network that connects different departments in any University or larger company is called a WAN.

MANs - Metropolitan Area Network, in essence a computer network that is meant to span a whole metropolitan area.

The Internet - the term "The Internet" is used to describe the network of computer networks installed across the globe. They are interconnected and most computers connect to it via the Internet Service Providers.